In modern transceiver circuits for use in mobile telephones, particularly in the bands for GSM and UMTS, it is frequently required to supply the output mixer stage of the HF-line of the transceiver (“open collector” or “open drain”) with a direct voltage externally via the HF-output pin. A so-called bias network provided for this purpose feeds the open-collector or open-drain output stages of the transceiver. This bias network should be integrated into a TX filter module. In order to maintain the size of the module as small as possible and thusly remain competitive, the normally used circuit technology cannot be employed in this case.
In order to feed a direct voltage into an HF-line, it is common practice to utilize a Bias-T connector element as described, for example, by James R. Andrews, entitled “Broadband Coaxial Bias Tees” (Picosecond Pulse Labs, Application Note AN-1e, Revision 1, Copyright November, 2000, http://picosecond.com/objects/AN-01e.pdf). The direct voltage is fed into the central conductor of a coaxial cable via a resistor or an inductor. In this case, the resistance should be significantly higher than the impedance of the HF-line in order to prevent a load on the HF-line. At a higher power demand, the direct voltage is coupled into the HF-line via an inductor in order to prevent a voltage drop from occurring at a resistor.
If the bias network is used in a TX (transmission) filter module, the module is preferably arranged on an LTCC substrate (low-temperature cofired ceramics), and the circuit of the bias network should be integrated into the LTCC substrate. When using a coil as an inductor for connecting the direct voltage, relatively high inductance values are required for achieving a sufficient decoupling of the direct voltage supply line from the HF-line. This complicates the integration of the inductor into the LTCC substrate if predetermined maximum dimensions of the TX filter module need to be observed.